The Cerberus Hammer is very powerful. It's a full auto version of the M-96 Mattock that doesn't lose any accuracy.

The Krysae Sniper Rifle (Turian anti-material rifle) is also crazy overpowered. My Shepard with the Cerberus Hammer, plus AI Garrus with the Krysae Sniper Rifle is almost too overpowered for the normal difficulty. We're crushing the Cerberus Atlas Mechs and Harvesters in like 5 seconds. I'm also level 41, so that may have something to do with it.

I'll echo the sentiments about the Harrier, assuming it's as OP in singleplayer as it is in multiplayer; it's basically a full-auto Mattock.

I played as every class on Insanity with the "stock" weapons and it was already pretty easy. Black Widow or N7 Valiant for Infiltrator, Mattock for everything else. AP ammo as extra power for every class but Infiltrator which I picked Energy Drain to combat shield gate (alternatively you could just take EDI/Garrus/Tali on every mission).

The only thing that's weird from a roleplaying perspective is that the Harrier has a big fat Cerberus logo on the butt of the rifle. It's kinda funny that Shepard is running around telling everybody that he doesn't work for Cerberus anymore but then is using one of their guns.

Maybe it's because I had to warm back up to it, but I found my first run of ME3 (it was either on Normal or Hardcore) to be noticeably more difficult than my insanity run so far. ME2 was actually insane on a regular basis (hence the name of the difficulty). ME3 hasn't been the least bit difficult in any way shape or form. I think they tuned it down to not be hard since a lot of people were like, "insanity is a bitch" for ME2. Shame they did it though. Insanity is supposed to be a bitch and most of the complaints were probably happy ones where people were having fun with it despite it being a kick in the balls.

Anyways, DLC weapons might help if they're better guns but it's not like playing with stock guns is going to be hard, especially if you've already beaten the game once and have gotten used to how combat works.